Faraday cup array integrated with a readout IC and method for manufacture thereof

ABSTRACT

A detector array and method for making the detector array. The array includes a substrate including a plurality of trenches formed therein, and includes a plurality of collectors electrically isolated from each other, formed on the walls of the trenches, and configured to collect charge particles incident on respective ones of the collectors and to output from said collectors signals indicative of charged particle collection. The array includes a plurality of readout circuits disposed on a side of the substrate opposite openings to the collectors. The readout circuits are configured to read charge collection signals from respective ones of the plurality of collectors.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by terms of ContractNNL-04-AA21A from NASA.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Application Ser. No. 61/036,844,filed on Mar. 14, 2008, entitled “HIGH DENSITY FARADAY CUP ARRAY OROTHER OPEN TRENCH STRUCTURES AND METHOD FOR MANUFACTURE THEREOF,” theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to a charged particle detector and methods forfabricating and using the electron detector.

2. Description of the Related Art

In general, a Faraday cup is regarded as a simple detector of chargedparticle beams. A faraday cup typically includes an inner cupconcentrically located within a grounded outer cup. Faraday cups areknown for their large dynamic range and ability to function in a widerange of environments, including atmospheric pressure. Well designed andshielded Faraday cups have been reported to measure currents down to10⁻¹⁵ A, corresponding to 10⁴ charged particles per second. Whileelectron multipliers are more sensitive, Faraday cup detectors providequantitative charge measurements with high precision and stableperformance. For instance, electron multipliers are susceptible todegradation over time due to sputtering of the electron conversionmaterial, and the gain of these detectors can vary depending on the massof the impending ions.

Faraday cup arrays designed for use in a mass spectrometer have beenpreviously built which included an array of MOS capacitors formed on theinterior of high aspect ratio deep etched trenches in n-type silicon. Inthose designs, the silicon between each cup served to electricallyshield cups from their neighbors, enabling low signal cross-talk. Lineararrays of 64, 128 and 256 cups at pitches of 150 μm and 250 μm have beenfabricated. The width spacing between the cups was typically limited to50 μm. Detector arrays have been fabricated where for ion detectionmetal strip electrodes or MOS capacitors were used.

The following references all of which are incorporated in their entiretyby reference describe this work and other background work.

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SUMMARY

In one embodiment of the invention, there is provided a detector array,including a substrate including a plurality of trenches formed therein,and includes a plurality of collectors electrically isolated from eachother, formed on the walls of the trenches, and configured to collectcharge particles incident on respective ones of the collectors and tooutput from the collectors signals indicative of charged particlecollection. The array includes a plurality of readout circuits disposedon a side of the substrate opposite openings to the collectors. Thereadout circuits are configured to read charge collection signals fromrespective ones of the plurality of collectors.

In one embodiment of the invention, there is provided a method formaking a detector array. The method forms in a substrate a plurality oftrenches across a surface of the substrate, forms in the plurality oftrenches a plurality of collectors, forms a plurality of readoutconnections on a side of the substrate opposite openings to thecollectors with the readout connections being configured to collectsignals from respective ones of the plurality of collectors, andconnects respective ones of the plurality of readout connections torespective ones of the plurality of the collectors.

In one embodiment of the invention, there is provided a system forcollecting charged particles. The system includes a charged particlesource configured to produce the charged particles and a detector arrayconfigured to collect the charged particles. The detector array includesa substrate including a plurality of collectors formed in the substrateand disposed in sequence across a surface of the substrate, a pluralityof trenches formed in the substrate to accommodate therein the pluralityof collectors, a plurality of readout circuits disposed on a side of thesubstrate opposite openings to the collectors. The readout circuits areconfigured to collect signals from respective ones of the plurality ofcollectors.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic showing one embodiment of the invention of asystem 10 for charged particle or photon detection;

FIG. 2A is a process schematic showing one method for the formation ofan integrated readout circuit detector array of the invention;

FIG. 2B is a process schematic showing a bonding aspect of the methodfor the formation of an integrated readout circuit detector array inFIG. 2A;

FIG. 2C is a process schematic showing a detector cup fabrication aspectof the method for the formation of an integrated readout circuitdetector array in FIG. 2A;

FIG. 2D is a SEM micrograph image of alternative circular trenchconfiguration to that shown in FIG. 1;

FIG. 2E is a SEM micrograph image showing a cross-section depicting thea silicon wafer bonded to a ROIC substrate;

FIG. 3 is a process schematic showing another method for the formationof a bottom-side readout pad for a detector array of the invention;

FIG. 4 is a process schematic showing another method for variousembodiments of the invention;

FIG. 5A is a schematic showing a landing pad according to one embodimentof the invention;

FIG. 5B is a flow chart depicting a similar process as shown n FIG. 2A;

FIG. 6A is a depiction of an alternative trench configuration to thatshown in FIG. 1 where circular collectors are arranged in a honeycombconfiguration;

FIG. 6B is a depiction of an alternative cubicle trench configuration tothat shown in FIG. 1 where cubicle collectors are arranged in a regulararray configuration;

FIG. 7 is a flowchart depicting according to one embodiment of theinvention a process for making a detector array;

FIG. 8A is a schematic of a triode electron source for one embodiment ofthe invention;

FIG. 8B is a schematic of an ion source using the triode configurationof FIG. 8A;

FIG. 9 is a SEM micrograph of an electron impact ions source for oneembodiment of the invention;

FIG. 10A is a schematic illustrating a process according to oneembodiment of the present invention to fabricate the exemplarymicrotriode ion source of FIG. 9.

FIG. 10B is an optical micrograph showing a top view of the exemplarymicrotriode ion source depicted in FIG. 9, prior to release of theanode, cathode, and grid from the underlying silicon substrate;

FIG. 10C is an electron micrograph of the exemplary microtriode ionsource depicted in FIG. 9; and

FIG. 11 is a schematic of an integrated ion source and detector arrayaccording to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to the microfabrication of Faraday cup arraysfor use as a charged particle or photon detection device. The detectordevice in one embodiment of the invention includes an array ofmicrofabricated Faraday cups, where each microfabricated Faraday cupacts as an electrically shielded collector of charged particles(electrons or ions).

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, FIG. 1shows one embodiment of the invention of a system 10 for chargedparticle or photon detection. The system 10 of FIG. 1 includes adetector array 20 including a substrate 22 having a plurality ofcollectors 24 formed in the substrate 22 and disposed in sequence acrossa surface of the substrate 22. FIG. 1 shows trenches 26 formed in thesubstrate to accommodate the collectors 24. As discussed below, invarious embodiments of the invention, the trenches are disposed in alinear arrangement (as shown here and below) and have high aspectratios. Indeed, while shown in FIG. 1 in a linear one dimensionalconfiguration, the invention is not limited to this configuration butrather can include in various embodiments staggered, non-staggered,elongated, non-elongated, circular, oval, honeycomb, and other geometricconfigurations.

Regardless of the trench configuration, in one embodiment of theinvention, the collectors 24 are respectively connected by way ofreadout connections (not shown) to a plurality of readout circuits 28disposed on a side of the substrate opposite openings to the collectors24. The readout circuits are configured to read signals from respectiveones of the collectors 24. Electrodes 30 connect respective ones of thereadout circuits 28 to respective ones of the collectors 24. For thesake of simplicity, only two collectors 24 connected to two readoutcircuits 28, having two electrodes 30 are shown in FIG. 1.

The trenches 26 can have widths ranging from 5 μm to 100 μm, and canhave lengths up to 10 mm. The trenches 26 can have an aspect ratioranging from 4:1 to 12:1. The collectors as a group can occupy more than80%, 90%, or 95% of a surface of the substrate 22. The trenches 26 canform a set of position sensitive detectors. A substrate wall between thetrenches can have a thickness less than 50 μm. As a result, the trenches26 can form a set of high density position sensitive detectors. In oneembodiment, as discussed in more detail below, the collectors 24 can anisolation resistance between adjacent ones of the collectors greaterthan 1×10¹⁰Ω.

The collectors 24 can be made of any conductive material including forexample copper, aluminum, gold, platinum, and tungsten or combinationsthereof. Besides the collectors, FIG. 1 shows a metal layer 32 patternedon the substrate 22 disposed in a vicinity of the collectors 24. In oneembodiment, the readout circuits are used for measuring the chargecollected in each collector 24 over time (integrated) or as a functionof time (instantaneous). The readout circuits 28 in one embodiment canbe included on another chip separate from the chip carrying the detectorarray 20 and attached thereto as described below. In one embodiment, themetal layer 32 serves as a ground reference and/or a suppression gridfor the detector array 20.

For example, a suppressor grid can be used in various embodiments toprevent secondary emission from the cup. A suppressor grid is a metaltrace that weaves between the Faraday cup collectors. A bias voltage canbe applied to the suppressor grid (for example by the readout circuits28) to prevent the escape of secondary electrons generated inside thecup. The suppressor grid can also serve as an energy filter for incomingcharged particles.

The system 10 of FIG. 1 in one embodiment can also include or beconnected to a charged particle source 50 which directs chargedparticles to the detector array 20 where the charged particles arecollected by the collectors 24 which act as individual electrodesmonitoring the charge accumulation thereon with time. In variousembodiments, the charged particle source can include an ion source or anelectron source or a combination thereof. In various embodiments, thecharged particle source can include a hot filament, microwave plasma, orother ion sources known in the art which provide an ion into a detectorregion. In one embodiment, the charged particle source can include anelectron-injector material or a photosensitive material 34 disposed in avicinity of the collectors, which emits an electron (or electrons) as acharged particle or as charged particles upon receiving light or x-rayor high energy particle thereon. For example, the collectors 24 shown inFIG. 1 could themselves contain a coating of photosensitive material orelectron injector material. Accordingly, the detector array 10 can be apart of a Faraday cup array, a magnetic sector ion detector, a detectorin scanning or transmission electron microscope, a charged particledetector, an x-ray detector, a photon detector, and/or a chemicalsensor.

In those embodiments, the detector array 20 serves as a positionalsensor regarding individual collector currents in time and in position.For example, in a magnetic sector field detector, ions emitted from anion source can be directed in a direction transverse to the longitudinalaxis of for example elongated collectors 24 and can be introduced into amagnetic field sector. In the magnetic field sector, the ions willtravel along trajectories in the magnetic field which depend on theircharge/mass ratio. Lower charge to mass ions are curved the most andwill arrive a position along the detector array which for example iscloser to the charged particle source than a higher charge to mass ions.The higher charge to mass ions will be incident on and then collected onfor example those collectors farther from the charged particle source.Similarly, in a detector in scanning or transmission electronmicroscope, the detector array also serves as a positional sensorregarding individual collector currents in time and in position.Electrons from the imaging optics are deflected according to theirkinetic energy such that lower energy electrons will be moresubstantially deflected than higher energy electrons. Here, the lowerenergy electrons will be incident on and then collected on for examplethose collectors closest to the charged particle source. In opticaldispersion devices, light will be diffracted at different anglesdepending on the wavelength. Lights of different wavelengths will beincident on different regions of the detector array 20. If an electronor charge emitting material on nearby or a part of the collectors, thenthe electrons or charge generated will be locally collected at nearbycollectors.

Further, the readout circuits 28 can collect and process the chargecollection information, or signals from individual ones of thecollectors 24, not only in a time coordinate (as discussed above) butalso in a spatial coordinate for position sensitive information, such asfor example in the magnetic sector field detector described above wherethe respective positions of the individual collectors 24 would berepresentative of different masses. The readout circuits 28 can beconnected to (or be otherwise in communication with) a microprocessor,memory, and a digital I/O port capable of generating control voltagessufficient to communicate and activate the collectors 24 and the variousmetal layers on the substrate surface. Moreover, the readout circuits 28by way of the microprocessor connection may exchange information tothose outside system 10. The microprocessor (not shown in FIG. 1) caninclude computer readable medium containing program instructions forexecution to process the data in a temporal and/or spatially integratedor instantaneous manner. The microprocessor may be implemented as ageneral-purpose computer system that performs a portion or all of themicroprocessor based processing steps of the invention in response toexecuting one or more sequences of one or more instructions contained ina memory. Such instructions may be read into memory from anothercomputer readable medium, such as a hard disk or a removable mediadrive.

The microprocessor can include at least one computer readable medium ormemory, such as the controller memory, for holding instructionsprogrammed according to the teachings of the invention and forcontaining data structures, tables, records, or other data that may benecessary to implement the invention. Examples of computer readablemedia are compact discs, hard disks, floppy disks, tape, magneto-opticaldisks, PROMs (EPROM, EEPROM, flash EPROM), DRAM, SRAM, SDRAM, or anyother magnetic medium, compact discs (e.g., CD-ROM), or any otheroptical medium, punch cards, paper tape, or other physical medium withpatterns of holes, a carrier wave, or any other medium from which acomputer can read.

In one embodiment of the invention, the Faraday cup arrays are made asone-dimensional or elongated Faraday cups. A variety of cup geometrieswhich can be fabricated ranging in width from 5 μm to 100 μm and havinglengths up to 4 mm. Larger ranges can be made with the same process. Areasonable minimum width would be 5 μm although there are no realrestrictions. Furthermore, the cup depth-to-width aspect ratios canexceed 8:1 using deep reactive ion etching technology. In someembodiments, thin silicon membranes between adjacent cups are less than5 μm wide and 100 μm tall.

In one embodiment of the invention, there is provided an array ofmicrofabricated Faraday cups, where each microfabricated Faraday cupacts as an electrically shielded collector of charged particles(electrons or ions). While not restricted to these examples, examples ofsuch microfabricated Faraday cup arrays are described in “METHOD OFMANUFACTURE FOR HIGH DENSITY FARADAY CUP ARRAY OR OTHER OPEN TRENCHSTRUCTURES” referenced above. In the process described in thatapplication, the array required multiple stages of interconnect toconnect to external measurement circuitry. First, the cups have to berouted to bondpads with metal fanout traces. These bond pads are thenconnected to an external package or measurement circuit by wire bonding.In the present invention, the readout circuits are integrated directlyunderneath the collector permitting more flexibility in designing thecollectors in terms of their spacing and shape, and also reducingcapacitance in the signal lines carrying collected current signal to thecircuitry processing the collected current signal.

FIG. 2A is a process schematic showing one method for the formation ofthe integrated readout circuit detector array of the invention. FIG. 2Bshows the wafer bonding and thinning process in more detail. FIG. 2Cshows the cup fabrication in more detail.

With reference to FIG. 2A, at 200, a readout integrated circuit (ROIC)wafer 50 having a plurality of readout circuits 28 (not enumerated) isprovided. ROIC wafers 50 can be commercially obtained through companiessuch as for example FLIR Systems, Inc. 70 Castilian Dr., Goleta, Calif.93117, USA. Custom ROIC wafers are available from Integrated SensorSolutions, 4104 Michael Neill, Austin, Tex. 78730. In one embodiment,the pattern of readout circuits is matched to the pattern of collectorsto be fabricated later and connected to the readout circuits.

Processes 210, 220, and 230 are shown in more detail in FIG. 2B. Withreference to FIG. 2A, at 210, the surface of ROIC wafer 50 is preparedfor bonding. The surface preparation depends on the particular type ofbinding process to be used. For example, borosilicate glass compounds orlower temperature spin-on-glasses compositions can be applied to thesurface of ROIC wafer 50. In some instances, adhesives such a glass,polymer, and/or silicone based adhesives can be used includingthermosetting and thermoplastic adhesives.

At 220, a substrate to have collectors formed therein (i.e., a collectorsubstrate) is bonded to a carrier substrate for wafer thinning. Thebonding here can use the above-noted glass binding techniques. Becausethere are no fabricated device circuits on the carrier substrate or thecollector substrate, higher temperature and or metallic bindingcompounds can be used. Alternatively, since removal of the carriersubstrate is later needed, the adhesive binding as discussed above canalso be a suitable bonding medium, allowing for removal of the carriersubstrate through dissolution of the adhesive by solvents or release byheating above the adhesive's temperature or thermosetting rating. Oncebonded, wafer thinning involving for example mechanical grinding,polishing, and/or chemical mechanical polishing techniques are used toreduce the collector substrate to a thickness equal to the desiredcollector depth. Standard wafer thinning for example can reduce thethickness of a silicon substrate to 10's to 100's of microns inthickness.

At 230, the thinned collector substrate is bonded to ROIC wafer 50. Thebonding can use for example any one of the bonding processes discussedabove. Once a bond has been made to the ROIC wafer 50, the bond to thecarrier substrate can be released and the carrier substrate removed, asshown in FIG. 2B.

Processes 240, 250, and 260 are shown in more detail in FIG. 2C. Withreference to FIG. 2A, at 240, the bonded thinned substrate is patterned,and collectors are formed therein. For example, trenches can be etchedin the thinned substrate using deep reactive ion etching (DRIE). Thetrenches extend through the depth of the thinned substrate and exposethe ROIC wafer 50. A standard silicon DRIE process using for example aninductively coupled plasma reactor can be used to form trenches ofaspect ratios (D/W) from 1:1 to 30:1. Trenches in the range of 4:1 to12:1 are prototypical of the invention.

Afterwards, at 250, a conformal insulator (e.g., parylene C) can bevapor deposited into the trench to serve as electrical insulation. Twoseparate insulator thicknesses (5800 Å and 10,900 Å) of parylene havebeen demonstrated as suitable for the invention. Parylene-C (PA-C) waschosen for the cup insulator because it is known to make very conformal,uniformly thick and pin-hole free films even in high aspect ratiofeatures. The cup insulator could also be fabricated by chemical vapordeposition of tetraethylorthosilicate (TEOS) which leads to conformalfilms of silicon dioxide. The conformal insulator is patterned to exposeunderlying metal connection pads to the readout circuits.

Afterwards, at 260, a conformal collector electrode layer (such as forexample Cu) is deposited for example by using metal-organic chemicalvapor deposition (MOCVD). Copper as a collector electrode material isdiscussed below, but other metals and silicides (or combinationsthereof) could be used for the collector electrode. Copper can bedeposited using for example hexafluoroacetylacetonate copper(I)trimethylvinylsilane, Cu(HFAC)(TMVS) as a Cu precursor at 200° C. and 1Torr. Cu(HFAC)(TMVS) as a Cu precursor is commercially sold asCupraSelect™ by Air Products and Chemicals. Inc. The conformal copperlayer fills the trenches and connects through the hole in the conformalinsulator to the ROIC substrate contact pads below. The copper alsodeposits on the top surface of the substrate between the trenches.

At 270, the copper on the top surface (which would electrically shortadjacent collector cups) is removed from the top surface of the wafer byargon ion milling. Argon ion milling can be performed for example at a30° angle to remove the surface copper without damaging the copper inthe cups. Other angles of incidence and inert gas ions are suitable forthe invention.

FIG. 2D is a SEM micrograph image of alternative trench configuration tothat shown in FIG. 1. As noted above, the invention is not limited toany particular trench or collector configuration, and can somewhatarbitrary pattern the trenches to any shape and configuration requiredor needed to match to the pattern of readout circuits on the underlyingROIC substrate. FIG. 2D shows a circular configuration. FIG. 2E is a SEMmicrograph image showing a cross-section depicting a silicon waferbonded to a ROIC substrate. While the trenches in FIG. 2E may not be ofan optimal size, depiction here shows the back-side integration of aROIC substrate to a trenched wafer structure.

FIG. 3 is a process schematic showing another method for the formationof the integrated readout circuit detector array of the invention. Atstep 300, a trench etch is performed on for example a thinned substrate80. At step 302, a thermal oxidation of the thinned substrate (forexample a thermal oxidation of a silicon substrate) is performed to formoxidized structure 82. At 304, the oxidized structure 82 is bonded to aROIC substrate (or a landing substrate as described above). At thattime, a conformal metal deposition follows to form the collectors 24.

FIG. 4 is a process schematic showing still another method for theformation of the integrated readout circuit detector array of theinvention. At step 400, a bonded, thinned substrate 90 (or for example asilicon-on-insulator wafer) is etched to form trench-etched structure 92for subsequent collector cup fabrication. In various embodiments of theinvention, collector widths range from 5 to 100 μm, cup lengths from 1to 4 mm, and cup-to-cup spacings from 5 to 25 μm. At step 402, a thermaloxidation of the trench-etched structure 92 (for example a thermaloxidation of a silicon substrate) is performed. At 404, the backsidesubstrate is thinned. At 406, the oxidized trench-etched structure 92 isbonded to a ROIC substrate (or a landing substrate as described above).Following bonding, patterning and etching can be used to open holes inthe residual amount of thinned substrate 90 in order to expose contactpads on the ROIC or landing substrate. At that time, a conformal metaldeposition follows to form collectors 24.

In one embodiment of the invention, the process described above in FIG.2A is used with a landing substrate substituted for the ROIC substrate.The process then follows with the landing substrate patterned forexample with a standard photolithography liftoff process with metalbeing provided to form a pattern of contact pads that will ultimately beconnected to the bottom of the cups of the collectors. The metallizationand photolithography liftoff process also forms a fanout metallizationpattern from the landing pads to contact pads at an edge of the landingsubstrate. In order to accommodate access to the contact pads, thelanding substrate is longer in one or more directions than the trenchedsubstrate in order to have the contact pads available around the edge ofthe landing substrate for electrical connection thereto. The contactpads would provide for connection to an external or integrated readoutcircuit for collection of charge information over time.

Accordingly, FIG. 5A shows a landing substrate 70 being provided thathas contact pads 72 (i.e., the landing pads) matched to the pattern ofcollectors to be fabricated later and that has lead lines 74 from thecontact pads 72 routed to peripheral positions 76 on the landingsubstrate 70. Optionally the contact pads could be routed to circuitryon landing substrate 70 for processing the collection current data fromthe collectors.

FIG. 5B is a flow chart depicting a similar process as shown n FIG. 2Abut given here for the purpose of illustrating the process for use ofthe landing substrate. At 510, the surface of a collector substrate isprepared for bonding. At 520, the collector substrate is bonded to acarrier substrate. At 530, the collector substrate is thinned substrateand trenched. For example, trenches can be etched in the thinnedsubstrate using deep reactive ion etching (DRIE) as described above. Thetrenches extend through the thickness of thinned substrate. At 540, thethinned collector substrate is bonded to a pre-patterned landingsubstrate (as discussed above). At 550, the carrier substrate is removedand processing of the collector substrate is completed. For example, aconformal insulator (e.g., parylene C) can be vapor deposited into thetrench to serve as electrical insulation. The conformal insulator ispatterned to expose all or a part of the landing pads and contact padson the landing substrate.

Afterwards, a conformal collector electrode layer (such as for exampleCu) is deposited. The conformal copper layer fills the trenches andconnects through the hole in the conformal insulator to the landing padsbelow. In one embodiment, the Cu forms the bottom the collector cupelectrode. The copper also deposits on the top surface of the substratebetween the trenches. Any copper on the top surface can be removed fromthe top surface of the wafer by argon ion milling.

FIG. 6A is a depiction of an alternative trench configuration to thatshown in FIG. 1 where circular collectors are arranged in a honeycombconfiguration. FIG. 6B is a depiction of an alternative cubicle trenchconfiguration to that shown in FIG. 1 where cubicle collectors arearranged in a regular array configuration.

FIG. 7 is a flowchart depicting a process for making the detector arraysof the invention. At 710, a plurality of trenches is formed in asubstrate across a surface of the substrate. At 720, a plurality ofcollectors is formed in the plurality of trenches. At 730, a pluralityof readout circuits is formed in a vicinity of the collectors with thereadout circuits being configured to read signals from respective onesof the plurality of collectors. At 740, respective ones of the pluralityof readout circuits are connected to respective ones of the plurality ofthe collectors.

At 710, the trenches can be formed by DRIE of silicon. The trenches canhave widths ranging from 5 μm to 100 μm and lengths up to 10 mm.Accordingly, the trenches can have an aspect ratio ranging from 4:1 to12:1. The trenches can occupy more than 80%, 90%, or 95% of a surface ofthe substrate.

At 720, the collectors can be formed on an aluminum metal, a coppermetal, and/or a metal silicide. At 730, a trace metal layer can bepatterned on the substrate between and around the plurality ofcollectors. The metal layer can function as a ground reference and/or asuppression grid for the detector array. Further, a readout circuit canbe connected to the metal layer for reading signals from respective onesof the plurality of collectors.

Faraday cups or similar detectors have also been used prior to theinvention as detectors in traditional magnetic sector mass analyzers.The Faraday cup serves as a single point ion detector and the magneticfield is scanned to collect particles of different mass. A detectorarray allows for simultaneous collection of all masses, leading to amore efficient detector. In one embodiment of the invention, the denserspacing of Faraday cups in the array as compared to previous arraysprovides improved accuracy and efficiency for example in ion detectorarrays in spectrometers, including spectrometers for isotope abundancemeasurements.

Faraday cups or similar collectors have also been used prior to theinvention as chemical sensors working close to atmospheric pressure anddetecting chemical agents based on ion mobility and differential ionmobility detectors. U.S. Pat. No. 6,809,313 (whose contents areincorporated herein by reference) describes the use of metal stripelectrodes, not true Faraday cups, for chemical sensors. In oneembodiment of the invention, the denser spacing of Faraday cups in thearrays of the invention as compared to previous Faraday cup arraysprovides for improved accuracy and efficiency for use in chemicalsensors and in ion mobility and differential ion mobility detectors.

Accordingly, in one embodiment of the invention, there is provided asystem (such as for example system 10 in FIG. 1) for charged particledetection. The system includes a detector array configured to collectcharged particles. The detector array includes (as discussed in detailabove) a substrate including a plurality of trenches formed therein, aplurality of collectors electrically isolated from each other. Thecollectors are formed on the walls of the trenches are configured tocollect charge particles incident on respective ones of the collectors.Adjacent ones of the plurality of trenches are disposed in a lineararrangement, although in other embodiments the trenches may bearbitrarily positioned and include two dimensional configurations. Thesystem can include a charged particle source (e.g., an ion source or anelectron source) for the generation of charged particles.

In one embodiment, the charge particle source can be an electron source102 or ion source 104 fabricated on a silicon substrate and utilizingfor example a carbon nanotube field emission electron source includingas shown in FIGS. 8A and 8B a cathode with aligned carbon nanotubes, acontrol grid, and an collector or extraction electrode. The collectorelectrode will be discussed below as the collector electrode permits oneto build and test an ion source before utilizing such a source as afree-standing or integrated part of a detector array. The extractionelectrode which contains a grid or slit will be used to provide a biasso as to extract ions from an ionization region after the control grid.FIG. 8A is a schematic of a triode configuration for one embodiment ofthe invention for an electron source 102. FIG. 8B is a schematic of anelectron-impact ion source 104 using the triode configuration of FIG.8A. FIG. 9 is a SEM micrograph of a triode that can be operated as anelectron source or an electron-impact ion source for one embodiment ofthe invention.

The generation of gas phase ions by electron impact is a commontechnique in the fields of mass spectrometry and vacuum science. In massspectrometry, electron-impact sources ionize gas phase analytes prior tomass separation and ion detection. In vacuum science, ion vacuum gauges,residual gas analyzers, and He leak detectors all operate usingelectron-impact ionization. Thermionic cathodes are reliable andeffective for many applications; however, the power consumptionassociated with heating these cathodes is a major limitation indeveloping miniature field-portable ion sources. In many emergingapplications such as field-portable mass spectrometers; the powerrequired to heat the thermionic electron source could exceed thecombined power requirements of all other system components. Therefore,field emission cold cathodes which nominally operate at room temperatureare attractive for some electron-impact applications. Workers haveevaluated a number of cold cathode materials including for examplediamond-coated silicon whiskers for application in an ion trap massspectrometer, carbon nanotubes (CNTs) and molybdenum tips as an electronsource in vacuum ion gauges, and integrated field emitters forelectron-impact ionization inside field emission displays. Additionalbenefits of field emission sources are the fast turn on and the abilityto run in a pulsed mode. Thermionic technology does not readily scaledown to microdevices, while field emission devices are naturallymicroscale and have the potential to generate larger emitted currentdensities.

FIG. 8A depicts a vacuum triode device with both the grid and the anodebiased positively with respect to the grounded cathode to provide anelectron source 102. FIG. 8B illustrates one embodiment of howelectron-impact ionization can be utilized with the same device to serveas an ion source 104. A positively biased grid affects the fieldemission of electrons from the cathode 112. Some percentage of theemitted electrons passes through the grid apertures 110 into the regionbetween the grid and the negatively biased ion collector 116. Theelectrons are decelerated by the collector bias and ultimately deflectedback towards the grid 110 if the collector voltage is large enough. Ifan electron-impact ionization event occurs in this region between thegrid 110 and the collector 116, the positively charged ion will beaccelerated towards the collector electrode. The collector electrode 116may contain a grid or a slit that enables these ions to pass through forexample to a detector array of the invention. If an electron-impactevent occurs in the region between the cathode 112 and grid 110, thegenerated ion will be accelerated into the cathode, possibly damagingthe electron emitters.

One illustrative fabrication process by which the ion source of FIG. 9can be made is described below. More details of the fabrication and thecharacterization are found in Bower et al, “On-chip electron-impact ionsource using carbon nanotube field emitters,” Applied Physics Letters90, 124102 (2007) published online Mar. 20, 2007, the entire contents ofwhich are incorporated herein by reference.

As described therein, polycrystalline silicon structures that form thedevice electrodes were initially formed parallel with the substratesurface and embedded in highly doped silicon dioxide. A MEMS foundrysuch as for example MEMSCAP Inc., Durham, N.C. was used for fabricationof the ion source. After the MEMS fabrication, the sacrificial silicondioxide was etched in hydrofluoric acid to release the electrode panels.The catalyst for CNT growth, in this example 50 Å of iron, wasselectively evaporated onto the cathode using an integrated shadow mask.The CNTs were grown using microwave plasma chemical vapor depositionwith ammonia/methane gas chemistry. Electron microscopy revealedmulti-walled CNTs with an average diameter of approximately 30 nm. TheCNT length was controlled by varying the growth time. After CNTdeposition, the panels were manually rotated and locked in place normalto the substrate using a tongue-in-groove MEMS technique. The device wasmounted and wire bonded to a ceramic board for testing. The specificdevice characterized here has a cathode-to-grid spacing of 50 μm beforeCNT deposition, a cathode-to-grid spacing of 30 μm after CNT deposition,and a grid-to-collector spacing of 280 μm. The cathode produced was a70×70 μm² panel and the grid produced was a 3×3 array of 20×20 μm²apertures, with a 2.5 μm grid wire. With this configuration, theelectric fields required to generate 1 nA and 1 μA of electron currentwere 5 and 6 V/μm, respectively. These devices were routinely capable ofgenerating field emitted electron current in excess of 50 μA.

A better understanding of the configuration, the fabrication, and thetesting results for the triode ion source described above will be hadwith reference to the following more detailed discussion. FIG. 8A is aschematic diagram of one embodiment of the triode of the invention,where V_(g) is the applied grid voltage, V_(a) is the anode voltage, andI_(a) is the measured anode current. According to one embodiment of thepresent invention, the ion source 104 ionizes molecules using eitherelectron impact ionization or field ionization. In the electron impactionization source of FIG. 8B, the grid electrode 110 is biasedpositively with respect to the cathode electrode 112 to cause electronemission. Electrons gain kinetic energy based on this voltage and canimpact ionize analyte ions in the gas in the vicinity of the gridelectrode 110. By way of contrast, in a field ionization source (whichthe invention can utilize as well and which FIG. 8A can be considered adiagram for provided the biases on the cathode and anode elements werereversed), the grid electrode 110 is biased negatively with respect tothe cathode electrode 112. The negative voltage does not promoteelectron emission but rather generates a high electric field (forexample about the carbon nanotubes 114 shown in FIG. 9 (and FIG. 10C)that can field ionize analyte species in the vicinity of the gridelectrode 110.

FIG. 10A depicts a process to fabricate an exemplary on-chip vacuummicrotriode that can be used to implement ion source 104. In thisillustrative, non-limiting process, a silicon dioxide layer 128 isthermally grown on a silicon substrate 130 serving as a support of theion source 104 and the ion collector 106. After which various layers ofa sacrificial oxide 132 are deposited using for example plasma enhancedchemical vapor deposition. In this illustrative process, a firstsacrificial oxide layer can be deposited, after which the firstsacrificial oxide layer is patterned to form holes 134 exposing theunderlying silicon dioxide. Polysilicon layer 136 can be deposited usingplasma enhanced chemical vapor deposition to cover the first sacrificialoxide layer and fill the holes 134 with polysilicon. The polysiliconlayer 136 is patterned to form the structures shown in FIG. 11Bincluding the grid pattern denoted in FIG. 11B and the tapered patternon the anode electrode 142. A second sacrificial oxide layer can then bedeposited over the entire structure. After which, both sacrificial oxidelayers are removed to release the polysilicon structures.

Carbon nanotubes 114 can then be formed on for example the cathodeelectrode 144 shown in FIG. 10B, using the techniques for carbonnanotube growth as discussed above. Afterwards, the polysilicon panelscan be rotated and locked into place, producing the structure shown inFIG. 10C.

During ionization testing of the triode of FIG. 9, a quantitativemeasure of the electron current (I_(e)) that passes into the ionizationvolume is unavailable because all of the emitted electrons areeventually captured by the grid. However, the measured grid current(I_(g)) should be proportional to the electron current (I_(e)) duringelectron-impact ionization (I_(g)αI_(e)). In a He atmosphere, theemitted electron current (measured at both 10⁻⁵ Torr and 50 mTorr) didnot exhibit a strong dependence on gas pressure. The ion current didincrease as He chamber pressure increased, as expected fromelectron-impact theory. At the chamber base pressure the measured ioncollector current is less than 10 pA, while at a pressure of 50 mTorrthe ion current approaches 100 nA, representing four orders of magnitudechange. The ion current began to saturate as the grid voltage wasincreased. Other gasses such as Ar and Xe also showed similarperformance, with these larger gasses exhibiting larger ratios ofcollector current to grid current.

These results show the viability of this on-chip ion source as an ionsource for a system utilizing the Faraday cup arrays of the invention.

FIG. 11 is a schematic of an integrated ion source and detector arrayaccording to one embodiment of the invention. In this embodiment, an ionsource such as ion source 104 is attached to a portion of the substrate202 removed from the trenches and collectors. Accordingly, FIG. 12depicts a detector array 200 including substrate 202 having a bondedwafer containing a plurality of trenches 204 disposed for example insequence.

A plurality of collectors (not explicitly shown in this depiction) aredisposed in the trenches 204. The collectors as in the other embodimentscan collect charged particles incident on respective ones of thecollectors and to output from the collectors signals indicative ofcharged particle collection. As shown in FIG. 12, the integrated ionsource 206 is attached to a portion of the substrate removed from thetrenches 204.

The ion source by way of grids 208 can direct ions across the detectorarray 200. For example, a magnetic field sector (not shown as themagnetic field lines permeate the structure shown in FIG. 12) candeflect the ions along different trajectories to impinge the ions ondifferent ones of the collectors depending on a charge-to-mass ratio ofthe ions.

Ion source 206 can include an electrode (e.g., a cathode) including acarbon nanotube as shown in FIG. 10C. The carbon nanotube can bedisposed on an electrode support spacing the carbon nanotube a distanceabove a surface of the substrate. The electrode as in other embodimentscan be configured to field ionize or electron impact ionize a gas phaseanalyte in a vicinity of the electrode. Grids 208 in ion source 206 canbe configured to be an acceleration grid directing ions across thedetector array 200. Ion source 206 can be configured as in otherembodiments to generate ion beams by selectively using electron impactionization or direct field ionization.

Numerous modifications and variations of the invention are possible inlight of the above teachings. It is therefore to be understood thatwithin the scope of the appended claims, the invention may be practicedotherwise than as specifically described herein.

The invention claimed is:
 1. A detector array comprising: a substrateincluding a plurality of trenches formed therein; a plurality ofcollectors electrically isolated from each other, formed on the walls ofthe trenches, and configured to collect charged particles incident onrespective ones of the collectors and to output from said collectorssignals indicative of charged particle collection; a plurality ofreadout connections connecting to respective ones of the collectors; aplurality of readout circuits disposed on a side of the substrateopposite openings to the collectors and connected to the plurality ofreadout connections, said readout circuits configured to read saidsignals from respective ones of the plurality of collectors indicativeof charged particle collection; and each of said readout circuitscorresponding to only one of said collectors.
 2. The array of claim 1,wherein the trenches comprise widths ranging from 5 μm to 100 μm andhaving lengths up to 10 mm.
 3. The array of claim 1, wherein thetrenches comprise an aspect ratio ranging from 4:1 to 12:1.
 4. The arrayof claim 1, wherein the plurality of collectors occupies more than 80%of a surface of the substrate.
 5. The array of claim 1, wherein theplurality of collectors occupies more than 90% of a surface of thesubstrate.
 6. The array of claim 1, wherein the plurality of collectorsoccupies more than 95% of a surface of the substrate.
 7. The array ofclaim 1, wherein the collectors comprise an array of position sensitivedetectors.
 8. The array of claim 1, wherein the collectors comprise ametal layer including at least one of copper, aluminum, gold, platinum,and tungsten.
 9. The array of claim 8, wherein the metal layer includesat least one of a ground reference and a suppression grid for thedetector array.
 10. The array of claim 1, further comprising: anelectron-injector material disposed in a vicinity of the collectors andconfigured to emit an electron as the charged particle upon receivinglight or x-ray thereon.
 11. The array of claim 1, wherein a substratewall between the trenches includes a thermal silicon dioxide layer. 12.The array of claim 1, wherein a substrate wall between the trenches hasa thickness less than 50 μm.
 13. The array of claim 1, wherein thecollectors have an isolation resistance between adjacent ones of thecollectors greater than 1×10¹⁰Ω.
 14. The array of claim 1, wherein theplurality of collectors comprises at least one of a Faraday cup array, adetector for a magnetic sector field detector, a detectors in scanningor transmission electron microscope, a charged particle detector, anx-ray detector, a photon detector, and a detector in an ion mobilityspectrometer.
 15. The array of claim 1, wherein the substrate comprisesa silicon substrate including the plurality of trenches formed therein.16. A method for making a detector array, comprising: forming in asubstrate a plurality of trenches across a surface of the substrate;forming in the plurality of trenches a plurality of collectors; forminga plurality of readout connections on a side of the substrate oppositeopenings to the collectors, said readout connections configured tocollect signals from respective ones of the plurality of collectorsindicative of charged particle collection; connecting respective ones ofthe plurality of readout connections to 1) respective ones of theplurality of the collectors and 2) readout circuits, wherein each ofsaid readout circuits corresponds to only one of said collectors. 17.The method of claim 16, wherein forming the trenches comprises: formingsaid trenches having widths ranging from 5 μm to 100 μm and lengths upto 10 mm.
 18. The method of claim 16, wherein forming the trenchescomprises: forming said trenches having an aspect ratio ranging from 4:1to 12:1.
 19. The method of claim 16, wherein forming the trenchescomprises: forming said trenches to occupy more than 80% of a surface ofthe substrate.
 20. The method of claim 16, wherein forming the trenchescomprises: forming said trenches to occupy more than 90% of a surface ofthe substrate.
 21. The method of claim 16, wherein forming the trenchescomprises: forming said trenches to occupy more than 95% of a surface ofthe substrate.
 22. The method of claim 16, wherein forming the trenchescomprises: leaving a substrate wall between the trenches of a thicknessless than 50 μm.
 23. The method of claim 16, wherein forming thecollectors comprises: forming collectors of at least one of copper,aluminum, gold, platinum, and tungsten.
 24. The method of claim 16,further comprising: patterning a metal layer on the substrate in avicinity of the collectors.
 25. The method of claim 16, wherein forminga plurality of readout connections comprises: bonding a readout circuitwafer including readout circuits to the substrate, said readout circuitsfor collection and processing of said signals indicative of chargedparticle collection.
 26. The method of claim 16, wherein forming aplurality of readout connections comprises: bonding a landing circuit tothe substrate, said landing circuit having landing pads for connectionto the collectors and contact pads for connection to readout circuitryfor collection and processing of said signals indicative of chargedparticle collection.
 27. The method of claim 26, wherein forming aplurality of readout circuits comprises: connecting said contact pads tosaid readout circuitry on the landing substrate.
 28. The method of claim26, wherein forming a plurality of readout circuits comprises:connecting said contact pads to said readout circuitry removed from thelanding substrate.
 29. The method of claim 16, wherein forming aplurality of trenches comprises: forming a thermal silicon dioxide layeron the trenches.
 30. A system for collecting charged particles,comprising: a charged particle source configured to produce the chargedparticles; and a detector array configured to collect the chargedparticles, and including, a substrate including a plurality of trenchesformed therein, a plurality of collectors electrically isolated fromeach other, formed on the walls of the trenches, and configured tocollect charge particles incident on respective ones of the collectorsand to output from said collectors signals indicative of chargedparticle collection, a plurality of readout connections connecting torespective ones of the collectors; a plurality of readout circuitsdisposed on a side of the substrate opposite openings to the collectorsand connected to the plurality of readout connections, said readoutcircuits configured to read said signals from respective ones of theplurality of collectors indicative of charged particle collection; andeach of said readout circuits corresponds to only one of saidcollectors.
 31. The system of claim 30, wherein the charged particlesource includes at least one of an ion source and an electron source.32. The system of claim 30, wherein the charged particle source includesan electron-injector material disposed in a vicinity of the collectorsand configured to emit an electron as the charged particle uponreceiving light or x-ray thereon.
 33. The system of claim 30, whereinthe charged particle source includes an electron-injector materialdisposed in the plurality of trenches and configured to emit an electronas the charged particle upon receiving a high energy particle thereon.34. The system of claim 30, wherein the detector array comprises atleast one of a Faraday cup array, a magnetic sector field detector, adetectors in scanning or transmission electron microscope, a chargedparticle detector, an x-ray detector, a photon detector, and a chemicalsensor.
 35. The system of claim 30, wherein the charged particle sourceincludes an ion source configured to direct ions across the detectorarray.
 36. The system of claim 35, further comprising: a magnetic fieldsector configured to deflect the ions along different trajectories so asto impinge the ions on different ones of the collectors depending on acharge-to-mass ratio of the ions.
 37. The system of claim 35, whereinthe ion source is configured to field ionize or electron impact ionize agas phase analyte in a vicinity of an electrode.
 38. The system of claim35, wherein the ion source comprises at least one acceleration gridconfigured to direct ions across the detector array.
 39. The system ofclaim 35, wherein the ion source comprises at least one of an electronimpact ionization source and a field ionization source.
 40. The systemof claim 39, wherein the ion source is configured to generate ion beamsby selectively using electron impact ionization or direct fieldionization.
 41. The system of claim 30, wherein the charged particlesource comprises at least one carbon nanotube.
 42. The array of claim41, wherein the carbon nanotube is disposed on an electrode supportspacing said carbon nanotube a distance above a surface of thesubstrate.
 43. A detector array comprising: a substrate including aplurality of trenches formed in the substrate and disposed in sequenceacross a surface of the substrate; a plurality of collectorselectrically isolated from each other, formed on the walls of thetrenches, and configured to collect charged particles incident onrespective ones of the collectors and to output from said collectorssignals indicative of charged particle collection; a plurality ofreadout connections connecting to respective ones of the collectors; aplurality of readout circuits disposed on a side of the substrateopposite openings to the collectors and connected to the plurality ofreadout connections, said readout circuits configured to read saidsignals from respective ones of the plurality of collectors indicativeof charged particle collection; each of said readout circuitscorresponding to only one of said collectors; and an ion source attachedto a portion of the substrate removed from the trenches.
 44. The arrayof claim 43, wherein the ion source comprises at least one accelerationgrid.
 45. The system of claim 44, further comprising: a magnetic fieldsector configured to deflect the ions along different trajectories so asto impinge the ions on different ones of the collectors depending on acharge-to-mass ratio of the ions.